Glasses in the form of powder (termed frits) have been used for many years as sealants and as coatings on glasses, ceramics, and metals. Frits capable of performing those functions at low temperatures, i.e., temperatures below 500.degree. C., however, are relatively uncommon. Commercially available glasses having that capability have, for the most part, been based upon the PbO-B.sub.2 O.sub.3 eutectic, the most notable of such glasses being the family of PbO-B.sub.2 O.sub.3 -ZnO frits widely used in television picture tube and other electronic sealing applications. One especially advantageous characteristic exhibited by those frits is their good flow prior to sealing. Nevertheless, those frits possess the disadvantage of containing high levels of PbO (typically 70-80% by weight). Because of its possible injurious impact upon human health and the environment, research has been undertaken to develop glasses free of lead. In addition, the resistance of those frits to attack by boiling water is only moderate, and the high density of the frits (.about.6.5g/cm.sup.3) can be deemed undesirable for certain applications.
Therefore, the primary objective of the present invention was to devise lead-free glass compositions demonstrating a T.sub.g no higher than about 425.degree. C. which, in the form of frits, would demonstrate flow properties at least comparable to those displayed by the above-described lead-containing frits, viz., good flow at temperatures not exceeding 475.degree. C. and preferably not exceeding 450.degree. C., and which would exhibit resistance to attack by water greater than that exemplified by the above-described lead-containing frits.
A specific objective was to devise such glass compositions which would be operable as sealing frits in television picture tube applications.